The present invention relates to pyroelectric ceramics and more particularly to a pyroelectric ceramic incorporating strontium and to methods for fabricating the ceramic.
Several figures of merit are used to compare the properties of pyroelectric materials: ##EQU2##
F.sub.i and F.sub.V determine the current and voltage responsivity respectively. F.sub.D determines the signal to noise ratio, where Johnson noise dominates. From these figures of merit favourable material characteristics are dependent on a high pyrocoefficient P=(dPs/dT) (where Ps=polarisation, T=temperature), a low volume specific heat capacity c and a low dielectric constant and loss, .epsilon. and tan .delta. respectively. Uniform response over a specified temperature and frequency range must also be considered in choosing a suitable material. In addition to the figures of merit, important features are the mechanical properties of the material and the ease with which the material can be produced and processed. Ceramics developed from PbZrO.sub.3 -PbTiO.sub.3 solid solutions exhibit several desirable qualities, not least of which is the way in which the properties may be adapted to suit a particular application, by adjusting the composition of the ceramic. Undoped PbZrO.sub.3 is antiferroelectric, the perovskite structure being based upon two interpenetrating sublattices, whose directions of polarisation are opposed, so there is no net polarisation in the antiferroelectric phase. When titanium is added, as titanium oxide Ti0.sub.2, making a solid solution of PbZrO.sub.3 -PbTiO.sub.3 the result is a ferroelectric ceramic with a perovskite structure in which some of the B site Zr.sup.4+ ions have been substituted by Ti.sup.4+ ions. The effect of increasing the concentration of Ti.sup.4+ is to increase the transition temperature T.sub.o, that is the temperature at which the spontaneous dielectric polarisation goes to zero, and to make the first order ferroelectric to paraelectric phase transistion behave more like a second order transition.
Addition of Ti.sup.+ 4 to the composition improves the uniformity of the detector response over a temperature range -40.degree. to 70.degree. C. Increasing Ti.sup.4+ concentration also engenders an increase in spontaneous polarisation. The quantity of Ti.sup.4+ chosen is governed by the transition temperature T.sub.o required. For a pyroelectric material operating over a given temperature range one requires a high transition temperature such that the material remains poled and shows a uniform response. T.sub.o must not be too high, however, as the figures of merit deteriorate at temperatures further away from the transition temperature. The pyrocoefficient is given by the gradient of the polarisation/temperature curve. For a first or second order transition this gradient increases as the Curie point is approached. The dielectric constant .epsilon. also increases as temperature is increased towards the Curie point, but a maximum value for the ratio P/.epsilon..sup.1/2 can be found to give an optimum value for the figure of merit F.sub.D. This is explained in an article by R.W. Whatmore and F.W. Ainger entitled "Piezoelectric Ceramics for uncooled Infra-Red Detectors" appearing in Advanced Infra-Red Sensor Technology. Proc. SPIE 395 pp. 261-6 (1983). Iron and niobium oxides were added in sufficient quantities to remove a ferroelectric rhombohedral to rhombohedral phase transition at about 50.degree. C.
The most directly controllable property is resistivity which is lowered if sufficient uranium ions U.sup.6+ are substituted onto the B site of the perovskite structure. The conductivity mechanism below 0.4 mole ratio % U.sup.6+ is thought to be controlled by lead Pb.sup.2+ vacancies acting as acceptor centres, holes being the majority charge carriers. Above 0.4 mole ratio % U.sup.6+ doping, however, U.sup.6+ acts as an electron donor and electrons are the dominant charge carriers. Off-valent U.sup.6+ also influence the dielectric properties possibly by stabilising the domain walls, thus reducing the dielectric constant and loss. (The mole ratio % referred to hereafter as mr %, of a ceramic constituent is defined as the number of gram-moles of the constituent per gram-mole of the ceramic).